El Niño, La Niña, & the Western US: Frequently
Asked Questions

Compiled by Kelly Redmond from various sources.
Updated June 16, 1998, to add emphasis to La Niña effects.

Questions

What is El
Niño?

What is La
Niña?

What is ENSO?

How Does El
Niño Affect Climate in the West?

How Does La
Niña Affect Climate in the West?

Are
Combined Effects More Likely

Are Big El
Niños Different from Average El Niños?

How Much
Confidence Can We Place in The Predictions?

Is There A
Tie between El Niño and Global Warming?

Are There Long-Term
Trends Associated with El Niño?

What is El Niño?

El Niño is a warming of the Pacific Ocean between South America
and the Date Line, centered directly on the Equator, and typically
extending several degrees of latitude to either side of the equator.
Coastal waters near Peru also warm. The warming is expressed as a
departure from long-term average ocean temperatures, which are
generally cool in the region, due to upwelling. El Niño is thus
associated with a slackening, or even cessation, of the cold upwelling
conditions which typically prevail in that area.

During a typical El Niño, the ocean warms 1 or 2 °C
above its climatological average. A strong El Niño can warm
by 3-4 °C over large areas, and even 5 °C in smaller regions.

Typically, El Niño is first noticed along the South American
coast around Christmas (hence the origin from Peruvian fishermen of
its Spanish name – "the child"). Farther west, in the open
ocean, El Niño typically begins to appear about a month later
(near the Galapagos) to about 4 months later (near the Date Line)
than near the coast.

What is La Niña?

La Niña is essentially the opposite of El Niño. La
Niña exists when cooler than usual ocean temperatures occur on
the equator between South America and the Date Line. The name La
Niña ("the girl child") was coined to deliberately represent
the opposite of El Niño ("the boy child"). The terms El
Viejo and anti-El Niño are also sometimes used. La
Niña occurs almost as often as El Niño, but has been
lesser known. La Niña and El Niño are but two faces of
the same larger phenomenon.

Stronger than usual trade winds accompany La Niña. These
winds, from the east, push the ocean water away from the equator in
each hemisphere. (This is caused by the rotation of the earth.) Cold
water from below rises to replace the warm surface water which has
moved away from the equator.

The cool water acts as an impediment to the formation of clouds and
tropical thunderstorms in the overlying air. This suppression of
rain-producing clouds leads to dry conditions on the equator in the
Pacific Ocean from the Date Line east to South America.

What is ENSO?

ENSO stands for "El Niño / Southern Oscillation". The acronym
arose in the climate research community, and reflects an attention
bias toward the warm phase of the entire cycle.

El Niño is just one phase of an irregular fluctuation between
warmer than usual and colder than usual ocean temperatures in the
region mentioned above. The cold phase has recently come to be known
as "La Niña". The El Niño / La Niña "cycle"
does not occur with strict periodicity. Historically, an El
Niño usually recurs every 3-7 years, as does its (cold) La
Niña counterpart.

The overlying atmosphere is tightly coupled to ocean temperatures
and circulation patterns. An atmospheric pressure signal is seen
throughout the tropics that is strongly linked to El Niño and
La Niña. When barometric pressure is higher than usual in the
western Pacific near Indonesia, pressure is lower than usual in the
subtropical Pacific near Easter Island and Tahiti. This global-scale
pressure signal, identified 70 years ago, is known as the "Southern
Oscillation". Surface barometric pressure at Darwin, Australia, and
the island of Tahiti are strongly anti-correlated: when one is higher
than usual, the other is lower than usual. The difference, Tahiti
minus Darwin, suitably normalized, is referred to as the Southern
Oscillation Index (SOI), and is frequently used as a convenient,
simple and reasonably accurate tool to monitor the status of El
Niño / La Niña.

Because more attention has been devoted to El Niño, and noting
the association between the Southern Oscillation in the atmosphere
and El Niño (and La Niña) in the ocean, the research
community began to refer to the combination as ENSO (El
Niño / Southern Oscillation). This moniker is somewhat
asymmetric: El Niño pertains to just one of the two phases of
the Southern Oscillation.

It would be perhaps more accurate to refer to El Niño as the
warm phase of the Southern Oscillation, and to La Niña as the
cold phase of the Southern Oscillation. However, the term "ENSO" is
firmly engrained.

How Does El Niño Affect Climate in the West?

Winter versus Summer: The most unambiguous signal is
seen in the winter half-year, typically from October through March or
April. A weaker signal may be seen in some parts of the West in
summer or early autumn. Most of this discussion concentrates on
winter.

Tropical Storms: Eastern Pacific autumn tropical
storms, west of Mexico, appear to be less frequent in El Niño
years, a tendency which is well-established in the Atlantic. However,
those tropical storms that do occur have a greater than usual
tendency to recurve into Mexico or the southwest U.S. Higher than
usual water temperatures off the Mexican coast in El Niño
years can help maintain their strength, or cause them to be stronger
than they would otherwise be. Hurricanes need water temperatures of
about 27 °C (81 °F) or more to sustain themselves.

Winter Circulation: During El Niño years, the
storm track more frequently splits into two preferred branches. The
Aleutian Low, in the Gulf of Alaska, is deeper than usual, and one
branch of the jet stream departing from its vicinity heads toward the
Queen Charlotte Islands and the southern coast of the main part of
Alaska, bringing mildly increased storminess to those areas. A
second branch of the jet stream is seen across the southern tier of
the U.S. and northern Mexico, and with higher speeds than usual.
Storms approaching the Pacific Northwest, and southwest Canada, are
often split and weakened as they approach the shore, as their energy
is shunted toward the north and/or the south.

Winter Precipitation: With El Niño, the period
October through March tends to be wetter than usual in a swath
extending from southern California eastward across Arizona, southern
Nevada and Utah, New Mexico, and into Texas. There are more rainy
days, and there is more rain per rainy day. El Niño winters
can be two to three times wetter than La Nina winters in this region.

In the Pacific Northwest, El Niño tends to bring drier winters.
The area affected in this manner includes Washington, Oregon, and the
more mountainous portions of Idaho, western Montana and northwest
Wyoming. This area of influence extends well up into Canada, and
coincides very well with the Columbia River Basin on both sides of the
U.S. / Canada border.

In between these regions, including central and northern California,
northern Nevada, southern Oregon, northern Utah, southern Wyoming,
and much of Colorado, the effects of El Niño are ambiguous.
No strong association in either direction (toward wet or dry) can be
discerned.

Farther north, from the Queen Charlotte Islands around toward Kodiak
Island, the relationship again switches sign, and southern Alaska
tends to have wetter winters with El Niño.

In Hawaii, El Niño tends to bring dry winters. Drought is more
likely during El Niño years, during the October-March period.
This association is well known in the Hawaiian Islands.

In general, in all these regions, La Niña climate effects are
approximately, but not exactly, opposite to El Niño climate
effects.

Winter Temperature: Winter temperatures with El
Niño conditions tend to be warmer than usual from Washington
and northern Oregon across the northern tier to Montana, and also
along the West Coast. Conversely, cooler than normal temperatures
are seen in the far southeastern portion of the West, especially in
southeastern New Mexico.

Snowpack: With El Niño conditions,
precipitation and temperature effects combine to accentuate the
effect on snowfall. In the Southwest, there is a slight tendency
toward cooler winters, and a strong tendency toward wet winters,
which makes higher elevation snowpack deeper. In the Pacific
Northwest, El Niño winters are warmer and drier than usual, so
that at a given elevation 1) less precipitation occurs, and 2) the
freezing level is higher, so the type of precipitation is more likely
to be rain, and 3) the accumulation season is shorter. All three
conspire to produce a smaller snowpack accumulation by the end of
winter in the Pacific Northwest.

Streamflow Effects: Most streamflow in the West is
produced by melting snow in the spring (in general, about 75 percent).
At lower elevations rain can be an important component of streamflow.
El Niño effects on streamflow are magnified versions of the
effects on the climate elements. Because of hydrologic lags (snow
does not usually begin to melt until spring), the effects of El
Niño are typically delayed, in some cases for several months.
Thus, the effects of El Niño on streamflow may not be manifest
until late spring or summer. Usually, El Niño results in less
streamflow to the Pacific Northwest and greater streamflow in the
Southwest.

Likelihood of Floods: In southern California,
Arizona, southern Nevada, New Mexico, and southern Utah, almost of
the major flood episodes on mainstem rivers have occurred during El
Niño winters. None have occurred during La Niña
winters. The likelihood of flooding is considerably increased, but
flooding is not a guaranteed outcome.

How Does La Niña Affect Climate in the West?

To a first approximation, it appears that the consequences of La
Niña are nearly the opposite of El Niño in much of the
U.S., including the West. In the previous discussion of El
Niño effects, simply substituting the opposite words yields an
approximately correct description.

Exceptions to La Niña / El Niño
Opposition:

The La Niña climate signal in the West seems more reliable
than the El Niño signal. This is especially true in the
Southwest. El Niño generally brings wet weather there in
winter, but there are a number of exceptions. La Niña
brings dry winters to the Southwest, and there are no exceptions,
during the past 65 years. That is, La Niña brings much more
consistent consequences in the Southwest. In the Pacific Northwest,
this appears to be not as true. La Niña generally brings
cold, snowy, wet, active winters to the northern Cascades and the
northern Rockies. There are a few exceptions to this picture among
La Niña years. There appear, however, to be more such
exceptions in El Niño years, to the dry, mild winter pattern
these regions typically experience with El Niño.

For both El Niño and La Niñna, the north/south dividing
line between the opposing effects in the Southwest and the Pacific
Northwest extends as a zone from about San Francisco to Cheyenne,
Wyoming. The effects of both phases of ENSO are equally nebulous in
this region. The effects of larger El Niños extend farther
north along the West Coast, and the effects of La Niña can
extend south along the West Coast from the Pacific Northwest. Thus,
northern California can be the recipient of moisture from the
northern end of El Niño's effects, and the southern end of
La Niña's effects.

In the central Sierra there are no large-scale winter floods
associated with El Niño. All but one of the biggest floods
have occurred in La Niña winters. However, not all La
Niña winters have large floods, and many have small or
average winter flood peaks. Thus, La Niña opens the door
to, but does not guarantee, large scale rain-on-snow conditions
associated with the biggest Sierra floods – more so than El
Niño. The deep tap to abundant tropical moisture (the
so-called "pineapple connection") associated with major Sierra
floods has a higher likelihood of occurrence in La Nina years than
in El Niño years, but in both cases is not common. These
very large floods can be generated in just a few days, and the
weather pattern during that time may poorly represent the overall
character of the winter. Both 1996-97 and 1985-86 illustrate this
point very well: without the short period of intense rains, these
two years with the largest floods would likely have entered the
record books as drought winters.

The overall atmospheric flow patterns differ substantially between
La Niña and El Niño winters. El Niño winters
tend to feature strong and persistent flow from the Pacific into
North America, blocking the movement of cold Canadian air toward
the south. La Niña winters have much more north-south
movement of air masses, and alternations of temperature,
particularly in the northern half of the West.

Although El Niño has received considerably more attention
than La Niña, evidence suggests that the types of weather
associated with La Niña winters have more deleterious
effects to the national economy than do those of El Niño.

Are Combined Effects More Likely?

In some areas, combined events can interact to accentuate the effects
of El Niño. For example, in the southern West, during the
winter months vigorous storms are more likely, precipitation amounts
are heavier, the frequency of precipitation is higher, and events are
more likely to persist. Thus, ground saturation is likely to be
greater, and landslides are more frequent. With more frequent and
vigorous storms, coupled with wet soils, trees are more likely to
topple. In the northern West, by contrast, such circumstances are
proportionately less likely.

Are Big El Niños Different from Average El Niños?

There is some evidence that the effects of large El Niños
(which constitute a small sample) may be different from those of the
typical El Niño. In particular, heavier precipitation may
occur farther north in Nevada and California, and especially along
the coast. For example, the record El Niño of 1982-83 brought
heavy precipitation as far north as Oregon and Washington, the only
major exception in the last 70 years to the typical dry winter
response expected in the Pacific Northwest.

How Much Confidence Can We Place in The Predictions?

All forecasts are fallible(!). In the Southwest, not all El
Niños bring wet winters, and in the Northwest, not all El
Niños bring dry winters. The most appropriate way to use these
forecasts is to "hedge one's bets" in the indicated direction. In
the Southwest, typically the likelihood of a wet winter is increased
from 50 percent (a coin toss) to about 65-75 percent likely. In the
Pacific Northwest and northern Rockies, for a typical El Niño
the likelihood of a dry winter is increased to 65-75 percent. For
the 1997-1998 El Niño, these likelihoods are even higher than
usual in the most affected areas, as much as 80 percent for a dry or
wet winter, in the two respective areas.

Is There A Tie between El Niño and Global Warming?

This is a matter of considerable speculation in the climate research
community. It is plausible that a warmer earth would produce more
and stronger El Ninos. There is some evidence that the earth has
warmed over the past two decades, and there is no doubt that El
Niño has been much more frequent in that time. If the
evidence of a warming earth is taken at face value (not universally
accepted), there still remains a wide spectrum of opinions on whether
we are seeing a manifestation of human modification of global
climate, or whether the natural climate system would be exhibiting
this behavior anyway.

Are There Long-Term Trends Associated with El Niño?

Yes. A number of climate indicators were noted to have changed in 1976,
especially around the Pacific Basin. Prior to 1976, El Niño
and La Niña occurred with about equal frequency, each at
intervals of about 3-7 years. Since 1976, there have been 9 El
Niños (using a 6-month average of the Southern Oscillation
Index of -0.50 as the criteria), or one every 2.2 years. There has
been just one moderate La Niña in that interval (1988-89) and
a rather weak La Niña (not even counted by some) in 1996-97.
Longer perspectives, since 1860, indicate that the 1976-1997 period
is quite unlike any other in the record. This is a source of
considerable puzzlement at this time.